About the Fuel Injector Size Calculator
This fuel injector size calculator works out the minimum flow each injector must deliver to support a target crank horsepower. Enter the horsepower goal, the engine’s brake-specific fuel consumption (BSFC — pounds of fuel per horsepower per hour), the number of cylinders and the maximum injector duty cycle you want to run. It returns the required flow in lb/hr and cc/min, plus the total fuel flow the pump must supply.
Tuners use it when adding a turbo or supercharger, switching to E85, or building a high-horsepower engine. Typical gasoline BSFC is about 0.45–0.50 for naturally aspirated engines and 0.55–0.65 for boosted engines, which run richer for safety. E85 needs roughly 30–40% more fuel by volume, so its effective BSFC is around 0.65–0.85.
Injectors are rated at a specific fuel pressure (often 43.5 psi / 3 bar). If you run a higher pressure the injector flows more, so the calculator also shows the smaller rated size that would deliver the required flow at your operating pressure. Keeping duty cycle at 80–85% leaves headroom for hot days and fuel-pressure drop.
With the default inputs, the required flow per injector is 31.3 lb/hr. Change any value above to recalculate instantly.
How to use the fuel injector size calculator
- 1Enter your target crank horsepower.
- 2Choose a BSFC that matches the engine and fuel (higher for boost and E85).
- 3Enter the number of injectors and your maximum duty cycle (80% is a safe target).
- 4Enter the injector’s rated pressure and the pressure your system will run.
- 5Pick the next available injector at or above the rated size shown.
Formula and method
Horsepower multiplied by BSFC gives the total pounds of fuel the engine burns per hour at full power. Dividing by the number of injectors and the maximum duty cycle (the fraction of time each injector is open) gives the flow each injector must deliver.
Injector flow rises with the square root of fuel pressure, so an injector run above its rated pressure flows more and a smaller rated size will do. Conversion to cc/min uses the common gasoline rule of 10.5 cc/min per lb/hr (a density of about 0.72 g/cc); gallons per hour use the same density.
- HP
- Target crank horsepower
- BSFC
- Brake-specific fuel consumption (lb of fuel per hp per hour)
- duty
- Maximum injector duty cycle (decimal)
- P
- Fuel pressure across the injector
Worked examples
400 hp naturally aspirated V8
400 hp × 0.50 BSFC = 200 lb/hr of fuel. Spread over 8 injectors at 80% duty, each must flow 200 ÷ 6.4 = 31.25 lb/hr, about 328 cc/min — so a 36 lb/hr (≈380 cc) injector gives comfortable headroom.
600 hp turbo six-cylinder
A boosted engine at 0.60 BSFC burns 360 lb/hr at 600 hp. Six injectors at 80% duty need 360 ÷ 4.8 = 75 lb/hr each, or about 788 cc/min, so 80 lb/hr (≈850 cc) injectors are a typical choice.
Same 400 hp V8 running 58 psi fuel pressure
At 58 psi an injector flows √(58 ÷ 43.5) = 1.155 times its rating. The engine still needs 31.25 lb/hr, but an injector rated 31.25 ÷ 1.155 ≈ 27.1 lb/hr (284 cc/min) at 43.5 psi will supply it.
Frequently asked questions
What BSFC should I use for injector sizing?+
Use about 0.45–0.50 for naturally aspirated gasoline engines, 0.55–0.65 for turbocharged or supercharged gasoline engines, and roughly 0.65–0.85 for E85. When in doubt, use the higher number so you do not run out of injector.
How do I convert lb/hr to cc/min?+
For gasoline, multiply lb/hr by about 10.5 to get cc/min, or divide cc/min by 10.5 to get lb/hr. The exact factor depends on fuel density, which varies slightly with blend and temperature.
What duty cycle should injectors run at?+
Most tuners aim for a maximum of 80–85% at peak power. Above about 90% the injector barely closes between events, fuel delivery becomes inconsistent and there is no margin for hot weather or voltage drop.
How does fuel pressure affect injector flow?+
Flow changes with the square root of the pressure ratio. Raising pressure from 43.5 to 58 psi increases flow by about 15.5%, but very high pressure can make some injectors open slowly and the pump must work harder.
Do I size for crank or wheel horsepower?+
Use crank (flywheel) horsepower, because BSFC is defined at the engine. If you only know wheel horsepower, add an estimated drivetrain loss first — the WHP to HP calculator can help.